In standard resistance heating of a liquid, electrical current passes through a resistive heating element that converts electrical energy into heat. The heat conducts from the hot resistive heating element to the liquid, heating the liquid. This scheme is widely used in devices such as residential and commercial water heaters, appliances, such as dishwashers, and industrial processes. In heating water, the scheme has produced problems because the surface of the resistance heating element becomes much hotter than the liquid to be heated. This higher surface temperature causes chemicals and impurities in the liquid to react, to precipitate out of the liquid, and to adhere to the hot surface of the resistance heating element, forming a lime coating on its sheathing. Over time this lime layer builds up, and acts as a thermal insulator. Thus, the now insulated resistance element gets hotter, wasting energy. As it operates at an even hotter temperature the resistance element eventually burns out. In addition, in heating of the liquid with a standard resistance heater the electrical energy dissipated in the resistor has to first heat the resistance heating element, then the resistance element's sheathing, then any lime buildup on the element's sheathing surface, and then finally the liquid. Thus, the heating of the liquid comes after some delay.
To address these problems, the lime coating on the resistance heater may be periodically removed from the appliance for deliming to prevent burn out and frequent replacement. The maintenance process of removing the mineral surface deposits takes time, adding cost and may use harsh chemicals which are damaging to the environment, costly and potentially dangerous.
Thus, better techniques for heating liquids are needed, and these techniques are provided in this patent application.